Function polynomial_sw2d 

fn polynomial_sw2d<const N: usize>(
    l: i32,
    m: i32,
    n: i32,
    z: Jet<N>,
    beta2: Jet<N>,
    azimuth: Jet<N>,
) -> Jet<N>

The inner sum S_lmn of the 2D spherical summand of reciprocal_term (upstream _s_sum_sw2d times e^(i m phi)), with z = k r_z, beta^2 = beta2 and beta e^(i phi) = (q_x + i q_y) / k: sum_s (-z)^(2n - s) beta^(l - s) e^(i m phi) / ((2n - s)! (s - n)! ((l + |m| - s) / 2)! ((l - |m| - s) / 2)!) over s = n..=min(l - |m|, 2n) with even l - |m| - s.